As optoelectronics progresses from discrete components to monolithic photonic integrated circuits, many building blocks will be utilized. One can envisage a single chip containing several active devices (various components) linked together with passive devices (for example waveguides). The technology by which the active components join the passive components, the Butt-coupling technology, is destined to be an important building block. At present there is no universally accepted technique to couple the optical devices. Although the general problem is one of integrating any device with any other device, a more common task is the integration of active devices and waveguides. Many techniques have been utilized to integrate active devices and waveguides on photonic integrated circuits, but each has its own problems.
The conventional method utilizes two growth steps to integrate an active device (for example a laser) with a waveguide. First, the active region of an active device is grown over all the substrate. The wafer is then removed from the growth apparatus, and the active device regions are masked and in regions where the waveguides are to be positioned the active device layers are removed by etching. The wafer is then returned to the growth apparatus for a second time during which the waveguide is grown in the etched regions, thus forming a butt-coupled waveguide for the active device. There are numerous problems associated with this method. The primary problem is that the interface between for example a laser and a waveguide is of poor quality and the shape of it is non-ideal. This is due primarily to growth problems at the interface during the step of growing the waveguide, and results in low coupling efficiencies and large reflections at the interface which are not ideal for most active devices. Moreover, this method is not favored for production because the process has numerous operations, many of which are difficult to reproduce. The cost of production also increases with the number of processing operations and growth steps, and hence a multiple operation and multiple growth method is not preferable.
Another proposed method, which has been demonstrated in GaAlAs materials, is to form a step in the substrate and to grow a structure over the step such that the active layer is aligned to the waveguide. This method is very attractive because it utilizes a single epitaxial step, however the coupling efficiency is small and the losses are large because the active layer is present in the passive section. In addition, this method has not been demonstrated in the InP/GaInAsP materials system and is unlikely in the near future to be demonstrated with any success.
Another alternative method is to grow the quantum well active region, mask the active regions and intermix the quantum wells such that they are transparent to the laser wavelength. There are several quantum well intermixing techniques such as impurity induced, impurity-free, implantation induced and laser induced intermixing. Today the Impurity-free vacancy disordering technique is fashionable in the literature. At present however the method has not been widely tested.
It would be desirable for there to be a method of integrating optical devices which does not suffer from the above-mentioned problems, and which can be carried out in a single epitaxial growth step.